FIN OH is for the colling fins on the back of the drive unit. Are you sure there is no other LED's showing alarm. It sounds strange that when you disconnect the motor from the drive that the alarm goes away because it has nothing to do with the motor.
In any case if you have swapped the drive units and the drive is still alarming out with the same alarm then it shows there is a problem in the drive. Probably there is a bad temperature sensor on the cooling fins in the drive.